Precipitation sensing system



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PRECIPITATION SENSING SYSTEM 8 Sheets-Sheet 8 Filed Dec. 19, 1960 7 360figg 364 366 f f3 0 372 E l5 g PULJE MV PUJE i AMPLIFIER AMPLIFIERPulsa" EAMPLIFIER [Elfen/UFE Jmes E. Murray' United States Patent OPRECIPITATION SENSING SYSTEM James E. Murray, Euclid, Fred H. Gutlr,`Warrensville,

James David Powell, Euclid, and Wendell Victor Peterson, South Euclid,Ghio, assignors to Thompson Ramo Wooldridge Inc., Cleveland, Ohio, acorporation of Ohio Filed Dec. I9, 1960, Ser. No. 76,884 36 Claims. (Cl.73-170) This invention relates to a weather instrument and moreparticularly to a precipitation sensing system which can be placed atany desired point to respond to falling precipitation and toautomatically produce electrical signals corresponding to the type ofprecipitation, whether it be hail, sleet, rain, drizzle, freezing rain,freezing drizzle, heavy rain or snow.

Various weather instruments are now available for measuring quantitiessuch as temperature, barometric pressure, relative humidity, winddirection, wind velocity, cloud height, etc. However, no satisfactoryinstruments have been available for signalling the type of precipitationwhich may be falling. It has been necessary to rely on human observationwhich in some respects is eX- tremely sensitive and selective but hasthe disadvantage in that human judgment as to the type of precipitationis not entirely reliable and reproduceable. This invention was evolvedwith the general object of eliminating the need for human observationand of providing a reliable system which would respond to fallingprecipitation and automatically produce output electrical signalsaccurately corresponding to the type of precipitation. With such asystem, it is possible to provide a completely automated weatherstation.

According to this invention, a system is provided which to a degreesimulates the operation of a human in sensing precipitation. The systemsenses various characteristics of falling precipitation whilesimultaneously sensing atmospheric conditions to produce a number ofcontrol signals. The control signals are fed to logic circuitry whichdevelops an output signal corresponding to the type of precipitation.Important features orf the invention reside in the general arrangementof the sensing devices and in the construction of the logic circuitry toprovide such output signals.

Another important feature of the invention is in the recognition of theneed for and the provision of a system for automatically producingcontrol signals corresponding to the kinetic energy of fallingparticles. 'Ihis system has many specific features which enable it toproduce such signals with a high degree of accuracy and reliability.Such signals are highly important in distinguishing between drizzle,rain, heavy rain and sleet.

A further important feature of the invention is in the recognition ofthe need for and lthe provision of a rebound sensing system whichautomatically produces control signals in response to the rebound ofparticles from a surface exposed thereto. The rebound sensing system ishighly important in distinguishing between precipitation in the form ofrain and solid precipitations such as sleet or hail.

Still another important feature of the invention is in the recognitionof the need for and the provision of a photo-optical sensing systemwhich develops output signals corresponding to the opticalcharacteristics of falling precipitation. The system of this inventionoperates to measure the reflectivity of falling particles. The controlsignals obtained are highly important as au aid in distinguishingbetween rain and snow.

A still further feature :of the invention is in the recognition of theneed for and the provision of a system for measuring the accumulation ofprecipitation on a sur- ICS face exposed thereto, todevelop outputsignals which are used in determining the presence of freezing rain orfreezing drizzle. The system of this invention is eX- tremely sensitiveand accurate in operation.

This invention contemplates other and more specific objects, featuresand advantages which will become more fully apparent from the followingdetailed description taken in conjunction with the accompanying drawingswhich illustrate a preferred embodiment and in which:

FIGURE 1 is a schematic diagram of a precipitation sensing systemconstructed according to the principles of this invention;

FIGURE 2 illustrates the construction of a mass accumulation sensingapparatus of the system` of FIGURE 1, and a mass accumulation sensorcircuit for developing output signals;

FIGURE 3 is a sectional view ott' a sensing plate o-f the apparatus ofFIGURE 2, taken substantially along line III-III of FIGURE 2;

FIGURES 4 and 5 are diagrams illustrating the form of signals obtainedin the apparatus of FIGURE 2, under different conditions of operation;

FIGURE 6 illustrates the construction of an impact sensor device of thesystem of FIGURE 1, together with the schematic diagram of an impactsensor circuit shown in block form in FIGURE 1;

FIGURE 7 is a cross-sectional view of the impact sensor device of FIGURE6, taken substantially along line VII-VII of FIGURE 6;

FIGURE 8 shows the construction of a rebound sensing apparatus of thesystem of FIGURE l and also shows a block diagram of the rebound sensorcircuit of FIGURE l;

FIGURE 9 shows the construction of a photo-optical sensing apparatus ofFIGURE l and also shows a block diagram of a photo-optical sensorcircuit of FIGURE 1;

FIGURE 10 is a schematic diagram of an AND circuit used at variouspoints in the system;

FIGURE 11 is a schematic diagram of an OR circuit used at various pointsin the system;

FIGURE l2 is a schematic diagram of an inverter ampliiier circuit usedat various points in the system;

FIGURE 13 is a schematic diagram of a preferred form of hip-flop circuitused at certain points of the system;

FIGURE 14 is a schematic diagram of a monostable multivibrator circuitused at various points in the system;

FIGURE 15 is a schematic diagram of a reset timer circuit used atvarious points in the system;

FIGURE 16 is a schematic diagram of a preferred form of amplitudediscriminator or squaring circuit which may be used at various points inthe system; and

FIGURE 17 is a schematic diagram of a pulse amplifier circuit which maybe used in the reset timer circuit of FIGURE 15.

Referring to FIGURE 1, the precipitation sensing system 10 of thisinvention generally comprises eight output circuits 11-18 selectivelyenergized through sensing and logic circuitry to indicate hail, sleet,drizzle, freezing drizzle, rain, freezing rain, heavy rain or snow. Theinputs of the output circuits 11-18 are connected to a logic circuitrysection 19 which has inputs connected to a plurality of sensor circuits,including a mass accumulation sensor circuit 20, an impact sensorcircuit 21, a rebound sensor circuit 22., a photo-optical sensor circuit23, a thunderstorm detector 24 and an air temperature sensor 25.

The mass accumulation sensor circuit 20 is connected to a sensingapparatus 26 and functions to develop output signals in response to theaccumulation or non-accumulation of ice on a surface, to distinguishbetween rain and freezing rain, or drizzle and freezing drizzle. Inbrief,

the apparatus 26 comprises a pair of plates 27 and 2S exposed toprecipitation and mounted on the legs of a tuning fork disposed within acovered tuning fork assembly 29. As ice accumulates on the plates 27 and28, the frequency of vibration of the tuning fork is lowered and bymeasuring the change in the frequency, an indication of massaccumulation is obtained. To remove loose snow and water from the plates27 and 23, a blower section 30 is provided, which also aids in periodicremoval of ice and in cooling the plates, after they are heatedperiodically by suitable means to remove ice. The circuit 29, theapparatus 26 and their operation are described in detail hereinafter inconnection with FIGURES 2-5.

The impact sensor circuit 21 is connected to a sensing device 31 and isused to develop signals which aid in clistinguishing between drizzle,rain, heavy rain and sleet. In brief, the illustrated device 31comprises a hemispherical plate 32 on which the precipitation falls,mechanically connected to a transducer within a housing 33. The circuit21 and device 3l are described in detail hereinafter in connection WithFIGURES 6 and 7.

The rebound sensor circuit 22 is connected to a sensing apparatus 34 andis used to develop output signals which are helpful in distinguishingbetween precipitation in the form of rain and solid precipitations suchas sleet or hail. In brief, the apparatus 34 comprises inclined plates35, 36, 37, and 38 on which the precipitation falls and from which itmay rebound against lower surface portions of an impact sensingstructure 39.

The photo-optical sensor circuit 23 is connected to a sensing apparatus4d and is used to develop output signals which aid in distinguishingbetween rain or drizzle and snow. In brief, the precipitation fallsthrough a central portion 41 of the apparatus 40, between portions 42and 43 thereof. Portion 42 houses a light source which projects lightthrough the portion 41 toward the portion 43 and a photoelectricdetector for measuring reflection of light from particles ofprecipitation falling tln'ough the portion 41. Portion 43 simplyprovides a dark background for contrast. It is found that snow has amuch higher degree of reflectivity than -rain and that a very reliableindication can be obtained in this fashion.

The thunderstorm detector 24 is of a type known in the art and itsconstruction and operation are therefore not described and illustratedin detail herein. It is used in providing an output indication whenthunderstorm conditions are present, which is helpful in determining thepresence of hail. It is also helpful with respect to providing anindication or non-indication of snow, since snow rarely occurs underthunderstorm conditions.

The air temperature sensor 25 is also of a type known in the art andhence is not illustrated and described in detail. Its output isconnected to a pair of amplitude discriminators 44 and 45. Amplitudediscriminator 44 provides an output signal when the temperature is inexcess of a certain value, for example 40 F., while amplitudediscriminator 45 provides an output indication when the temperature isin excess of another certain value, considerably lower, for example 15F.

LOGIC CIRCUITRY 19 (FIGURE l) The logic circuitry 19 operates toselectively energize one of the output circuits 11-18 in response toinput signals obtained from the sensor circuits Ztl-25. The controllinginput signals for the circuit 19 are produced on various conductors, asfollows:

A conductor 46, connected to the output of the mass accumulation sensorcircuit Ztl, has an output signal developed thereon, designated as M,when the accumulation of ice on the plate 27 exceeds a certain value, ina certain time interval.

A conductor 47, connected through an inverter 48 to the output of themass accumulation sensor circuit 20, has an output signal developedthereon, designated as when the accumulation of ice in a certain timeinterval on the plate 2'7 does not exceed a certain value.

A conductor 49, connected to one of four outputs of the impact sensorcircuit 21, has an output signal IY developed thereon under heavyprecipitation conditions, i.e. when the amount of precipitation in acertain time interval is in excess of a certain value.

Conductors 50, 51 and 52, connected to the other three outputs of theimpact sensor circuit 21, have output signals developed thereon inaccordance with the energy of impact of precipitation particles on thesensing plate 32. Conductor 5t) has an output signal IL developedthereon when the amplitudes of the impacts fall within a relatively lowrange of values. Conductor 51 has an output signal IM developed thereonwhen the amplitudes of the impacts fall within an intermediate range ofvalues, and conductor 52 has a signal IH developed thereon in responseto impacts having amplitudes exceeding a certain relatively high value.

Conductor 53, connected to the output of the rebound sensor circuit 22,has an output signal IR developed thereon in response to rebounds, abovea certain amplitude, on the apparatus 34.

Conductor 54 is connected through an inverter 55 to the output of therebound sensor circuit 22 and has a signal IR developed thereon inresponse to the absence of a rebound signal on the conductor 53.

Conductors 56, 57 and 58 are connected to outputs of the photo-opticalsensor circuit 23. Conductor 56 has a signal PL developed thereon whenthe reflection of light from the precipitation is in a relatively lowrange. Conductor 57 has an output signal PM developed thereon when thereflection of light is in a certain intermediate range, and conductor 58has an output signal PH developed thereon when the reflection of lightis in a certain intermediate range, and conductor 58 has an outputsignal PH developed thereon when the refiection exceeds a certainrelatively high value.

Conductor 59 is connected to the output of the thunderstorm detector andhas an output signal TS developed thereon under conditions indicative ofa thunderstorm.

Conductor 60 is connected through an inverter circuit 6l to the outputof the thunderstorm detector 25 and has a signal TS developed thereon inthe absence of conditions indicative of a thunderstorm.

Conductor 62 is connected to the output of the amplitude discriminator44 which, in turn, is connected to the output of the air temperaturesensor 25. A signal T40 is developed on the conductor 62 when thetemperature exceeds a certain value, for example 40 F.

Conductor 63 is connected through an inverter circuit 64 to the outputof the amplitude discriminator 44 and has a signal T40 developed thereonwhen the temperature is less than the aforesaid predetermined valuewhich may be 40 F.

Finally, conductor 65 is connected to the output of the amplitudediscriminator 45, connected to the air temperature sensor 25. A signalT15 is developed on the conductor 65 when the air temperature exceeds acertain relatively low value, for example 15 F.

The logic circuitry 1% will now be described with reference to thevarious conditions which cause selective operation of the outputcircuits lll-18.

(l) Hail Indication The input of the hail output circuit 11 is connectedto the output of an AND circuit 66 which develops an output signal inresponse to the concurrent application of input signals to all three ofits three inputs. These are connected to the conductors 62, 59 and 53.Accordingly, when there is concurrently a temperature in excess of acertain relatively high value, for example 40 F., thunderstormconditions and rebounds in the apparatus 34, a signal is applied throughthe AND circuit 66 to the hail output circuit 1l, energizing the same.

(2) Sleet Indication The input of the sleet output circuit 11 isconnected to the output of an AND circuit 67 which develops an outputsignal in response to the concurrent application of three input signalsto all inputs thereof. These inputs are connected to the conductors 52,63 and 53. Accordingly, the sleet output signal 12 is energized whenconcurrently there are high amplitude impact signals, rebound signalsand a temperature below a certain relatively high value, which may be onthe order of 40 F.

(3) Drizzle Indication The input of the drizzle output circuit 13 isconnected to the output of an AND circuit 68 which develops an outputsignal in response to the concurrent application of input signals toboth of its two inputs. One of the inputs is connected to the conductor47. The other input of the AND circuit 68 is connected to the output ofanother AND circuit 69, having one input connected to the conductor 50,and having another input connected to the output of still another ANDcircuit 70. Circuit 70 has three inputs connected to conductors 56, 65and 54. Accordingly, the drizzle output circuit is energized in responseto the concurent prence of signals on five conductors 47, 50, 54, 56 and65. Thus drizzle is indicated by the abensce of mass accumulation, bythe presence of lower amplitude impacts, by the absence of rebound, bythe presence of low intensity reflections and by the presence of atemperature in excess of a certain relatively low value, for example F.

(4) Freezing Drizzle Indication The input of the freezing drizzle outputcircuit 14 is connected to the output of an AND circuit 71 having twoinputs. The irst input is connected to the output of the AND circuit 69.The second input is connected to the conductor 46 which is connected tothe output of the mass accumulation sensor circuit. With thisarrangement, the freezing drizzle output circuit is energized inresponse to the same conditions as those which energized the drizzleoutput circuit 13, except that the freezing drizzle output circuit 14 isenergized in response to mass accumulation, whereas the drizzle outputcircuit 13 is energized in response to the absence of mass accumulation.

(5) Rain Indication The input of the rain output circuit 15 is connectedto the output of an AND circuit 72 having a rst input connected to theconductor 47 and a second input connected to the output of an ANDcircuit 73 having a first input connected to the output of the ANDcircuit 70 and a second input connected to the conductor 51. With thisarrangement, the rain output circuit is energized in response to theconcurrent development of signals on conductors 47, 51, 54, 56 and 65.Thus the rain output circuit is energized When there is an absence ofmass accumulation, when there are impacts having amplitudes greater thana certain intermediate value and less than a certain high value, whenthere is an absence of rebound, when the reflection from theprecipitation is within a certain relatively low range, and when thetemperature exceeds a certain relatively low value which may be 15 F. v

(6) Freezing Rain Indication The input of the freezing rain outputcircuit 16 is connected to the output of an AND circuit 74 having aiirst input connected to the output of the AND circuit 73 and a secondinput connected to the conductor 46. Thus the freezing rain outputcircuit is energized in response to the same conditions which causeenergization of the rain output circuit 15, except that the freezingrain output circuit is energized in response to the presence of massaccumulation, as indicated by a signal on the conductor 46, whereas therain output circuit 15 is energized in response to the absence of massaccumulation, as indicated by a signal on the conductor 47.

(7) Heavy Rain Indication The input of the heavy rain output circuit 17is connected to the output of an AND circuit 75 having a first inputconnected to the output of the AND circuit 72 and a second inputconnected to the conductor 49. With this arrangement, the heavy rainoutput circuit 17 is energized in response to the same conditions whichenergized the rain output circuit 15, provided there is also present asignal on the conductor 49. As described above, a signal is developed onthe conductor 49 when there is a large amount of precipitation.

(8) Snow Indication The input of the snow output circuit 18 is connectedto the output of an OR circuit 76 having a first input connected to theoutput of an AND circuit 77 and a second input connected to the outputof an AND circuit 78. The OR circuit 76 develops an output when there isan output from either of the circuits 77, 78. The AND circuit 77 isconnected to develop an output signal in the presence of snowflakeswhile the AND circuit 78 is connected to develop an output signal inresponse to the presence of snow pellets.

In particular, circuit 77 has four inputs connected to conductors 54,58, 60 and 63. Thus an output signal is developed by circuit 77 whenthere is an absence of rebound, when the reliection of light from theprecipitation is above a relatively high value, when there is an absenceof thunderstorm conditions, and When the temperature is below a certainvalue which may be 40 F. Such conditions prevail when the precipitationis in the form of snowflakes.

The circuit 78 has four inputs. The first three inputs are connected toconductors 50, 60 and 63. The fourth input is connected to the output ofan OR circuit 79 having inputs connected to conductors 57 and 58. Thusthe circuit 78 develops an output when the precipitation producesimpacts having amplitudes above a certain relatively low value, whenthere is an absence of thunderstorm conditions, when the temperature isbelow a certain value which may be 40 F., and when the reflection oflight from the precipitation is either in an intermediate range or abovea relatively high value. Such conditions prevail when the precipitationis in the form of snow pellets.

It should be noted that the snowflake indicating circuit 77 and the snowpellet indicating circuit 78 may be connected to separate outputcircuits, rather than through the OR circuit 76 to the single outputcircuit 18.

It may further be noted that the specic design of the system was basedupon limited experience with respect to weather conditions which prevailin connection with the various forms of precipitation in the Cleveland,Ohio, area. In other parts of the country, and with additionalexperience, the exact conditions of operation may have to be varied tosome degree. However, the general principle of operation remains thesame.

Mass Accumulation Sensor FIGURE 2 shows the mass accumulation sensorcircuit 20 and also shows the construction of the apparatus 26, in aplan View of the apparatus with the cover plates of the tuning forkassembly 29 being removed. As shown, the tuning fork assembly 29comprises a tuning fork 80 having a pair of leg portions 81 and 82 and abase portion 83 secured by a bolt 84 to a support 85. The plates 27 and28 are secured to the ends of the legs 81 and 82 through a pair ofcoupling members 86 and 87. The coupling members 86 and 87 arepreferably of a lightweight rigid material such as plastic. The legs 81and 82 vibrate horizontally, in a direction parallel to the plane of theplates 27 and 28, the natural resonant frequency of vibration beingdetermined in part by the mass presented by the plates 27 and 28together with the coupling members S6 and 07. It will be appreciatedthat as ice accumulates on the plates 27 and 28, the natural resonantfrequency of vibration will be lowered.

To vibrate the legs 81 and 82 of the tuning fork S0 and to deriveelectrical signals therefrom, a pick-up coil 88 is disposed adjacent theouter surface of the leg 81 While a drive coil 89 is disposed adjacentthe outer surface of the leg S2. The coils 58 and S9 are supported onuprights 90 and 91, secured to the support 85.

The assembly is mounted within a generally rectangular housing 92 whichincludes an intermediate generally vertical wall 93 approximately in theplane of the inner sections of the plates 27, 28 and coupling members 86and 07. The wall 93 has openings just large enough to allow passage ofthe plates 27, 2S and the ends of the coupling members 36, 37therethrough and to allow the required movement thereof, so that theWall 93 serves to minimize the entry of precipitation into the tuningfork chamber.

As above described, a blower is mounted within the section 30 of theapparatus 26. The blower may be of any desired construction. An outlettherefrom is provided in a Wall portion 94 of the housing 92 with adellector plate 95 being located on the wall 94 above the openingtherein and above the plane of the plates 27, 28. The blower directs astream of air across the plates with suflicient force to keep snow fromcollecting on the plates during time intervals when the apparatus isused to measure ice accumulation. The blower also serves to aid inremoving accumulated ice and in cooling the plates after they are heatedto remove accumulated ice. An electrical heater is preferablyincorporated in the blower.

Heating of the plates 27 and 2S is desirable for rapid removal of icetherefrom. It will be appreciated that it is necessary to periodicallyremove ice from the plate, to keep the output of the sensor up to date.

As shown in the cross-sectional view of FIGURE 3, the plate 27preferably comprises a thin electrical heating element 96 sandwichedbetween two thin metal plates 97' and 98, preferably aluminum plates.With this construction, heat may be applied uniformly over the entiresurface of the plate, to rapidly melt ice. The plate 28 preferably hasthe same construction as the plate 27.

The pick-up and drive coils 80 and 89 are connected through conductors99 and 100 to terminals of driver and amplifier circuits 101. Theconstruction of such circuits, as well as the construction of the tuningfork assembly with its coils, is known in the art. The circuits 101develop an output electrical signal having a frequency corresponding tothe natural resonant frequency of the tuning fork with the attachedplates 27, 28 and coupling members 86, 87. The frequency of the signaldecreases as ice accumulates on the plates 27, 28. An A.C. source 102develops a reference signal whose frequency is equal to that developedby circuits 101 when there is no ice on plates 27, 28. Such signals areapplied to squaring circuits 103 and 104 to develop square Wave signals.When the two frequencies are the same but of arbitrary phaserelationship, they may have a form such as illustrated by lines 105 and106 in FIGURE 4. However, when the ice accumulates on the plate 27, thenatural resonant frequency is lowered, and the outputs of the squaringcircuits may have forms as indicated by reference numerals 107 and 100in FIGURE 5, the signal 10S being of the same form as 106 While thesignal 107 is of a lower frequency than the signal 105.

The outputs of the squaring circuits 103 and 104 are applied to aflip-flop circuit 109, the output of which is applied to an averagingcircuit 110. Referring to FIG- URE 4, the flip-flop circuit may be setby each leading edge in the signal 105 and reset by each leading edge inthe signal 106 to develop an output signal having a form as indicated byreference numeral 111 in FIGURE 4. It will be noted that a series ofpulses of equal amplitude are produced. When such pulses are applied tothe averaging circuit 110, a substantially constant output signal 112 isproduced. It is noted that FIGURE 4 illustrates the operation with aphase relationship a1'- bitrarily assumed to be about With a differentphase relation, the width of the output pulses would be changed, and theaverage output would be changed, but would remain at a substantiallyconstant value.

Referring to FIGURE 5, when the flip-flop circuit 109 is set by eachleading edge of the signal 107 and reset by each leading edge of thesignal 108, the flip-dop circuit 109 produces an output signal having aform as indicated by reference numeral 113 in FIGURE 5. lt will be notedthat the width of the pulses varies in a certain fashion, dependent uponthe difference in frequency. When the output signal 113 from theflip-flop circuit 109 is applied to the averaging circuit 110, an outputsignal is developed having a form such as indicated by reference numeral114 in FIGURE 5.

The output of the averaging circuit is applied to a multivibrator 115,the output of which is applied to an averaging circuit 116.Multivibrator 115 is triggered by output signals from the averagingcircuit 110 having a certain amplitude and rate of change, and themultivibrator 115 stays on for a certain time intermal after triggeringthereof.

When the output of the averaging circuit 110 is substantially constant,as is true under equal frequency conditions of operation as illustratedin FlGURE 4, the multivibrator 115 is not triggered and its output staysat zero, as indicated by the straight line 117 in FIGURE 4. The outputof the averaging circuit 116 is also Zero, as indicated by the straightline 118 in FIGURE 4. When, however, there is a difference in thefrequencies, as illustrated in FIGURE 5, the multivibrator 115 isperiodically triggered at a rate corresponding to the difference infrequencies to produce an output signal of a form such as indicated byreference numeral 119 in FIGURE 5. The averaging circuit 116 thenproduces an output signal such as indicated by reference numeral 120 inFIGURE 5. It will be appreciated that since the frequency of the outputpulses from the multivibrator 115 increases with the difference infrequency of the applied signals, the output of the averaging circuit116 also increases in proportion to the difference in frequency.

The output of the averaging circuit 116 is applied to an amplitudediscriminator 121 which produces an output signal when the output fromthe averaging circuit 116 exceeds a certain value. The output of theaveraging circuit 116 may also be connected to `a terminal 122 which isnot used in the illustrated system, but may be used Where an analogsignal of mass accumulation is desired. In the event a digital approachto the measurement of mass accumulation is desired, the output of themultivibrator 115 may be applied to a counter which counts for a fixedtime interval. The count at the end of the timing interval would then bea digital representation of the difference in frequency between the twotuning forks and thus an indication of the actual mass accumulation. Therate of mass accumulation may be measured by measuring the rate ofchange of the frequency appearing at the output of the one-shot ormonostable multivibrator 115. In such cases, it may be desirable to usea relatively long accumulation period on the order of hours, rather thanminutes as is preferably used in the system 10.

The output of the amplitude discriminator 121 is applied to one input ofa dual input AND circuit 123, the output of which is applied to astorage circuit in the form of a flip-flop circuit 124. The output ofthe flipop circuit 124 constitutes the output of the mass accumulationsensor circuit, and is connected to the conductor 46, shown in FIGURE l.

The second input of the AND circuit 123 is connected to the output of anamplitude discriminator 125, the input of which is applied to a atsurface temperature sensor 126. The purpose of this arrangement is toavoid the possibility of an erroneous indication, which may be producedif the temperature of the collector plates 27, 28 is lower than freezingwhile the temperature of a standard reference surface is above freezing.The plates 27 and 28 may have a temperature somewhat lower than that ofa standard reference surface due to the operation of the blower, and dueto the material (metal) thereof. To prevent the erroneous indication, atemperature sensor device is associated with a reference surface, asdiagrammatically illustrated by the block 126, and is connected to theamplitude discriminator 125 to produce an output signal only when thetemperature of the reference signal is below freezing temperature. Thusa signal can be applied to the ip-op circuit 124 only under suchconditions.

A timer and control circuit 127 is provided for cyclic energization ofthe various components of the system. The circuit 127 includes aterminal 128 connected to a terminal 129 of the driver and ampliercircuits 101 and a terminal 130 of the A.C. source 102, a pair ofterminals 131 and 132 connected to terminals of the heating elements ofplates 27 and 28, a terminal 133 connected to the other terminals of theheating elements, a pair of terminals 134 and 135 connected to theblower within the housing 30 and a terminal 136 connected to theflip-flop circuit 124. The circuit 127 also has a pair of terminals 137and 138 for connection to a suitable power source.

In operation of the circuit 127, a power is applied to the terminals 134and 135 to energize the blower. At this time, no power Aor signals areapplied from the remaining terminals 128, 131-133 and 136, so that thetuning fork system, the heating elements, and the ip-op circuit 124 areinactive. Freezing precipitation may then be accumulated on the plates27 and 28 for a certain time interval, preferably on the order of fiveminutes.

Control signals are then applied from terminals 128 and 135 to energizethe tuning fork and measuring system, including the flip-flop circuit124, and to produce an output signal from the flip-flop circuit 124 ifthe ice accumulation exceeds a value determined by the amplitudediscriminator 117 and if the temperature of the reference surface isbelow freezing to produce an output from the amplitude discriminator125. This operation may take place in a very short time interval.

The control signals from terminals 128 and 136 are then cut off, thesupply of power to the blower is cut off, and power is applied to theterminals 131-133 to heat the collector plates for a certain timeinterval which may preferably be on the order of 2.5 minutes.

Power is then cut olf from terminals 131-133 to deenergize the heatingelements, while power is again applied to the terminals 134, 135 toenergize the blower for a certain time interval, preferably on the orderof 2.5 minutes. The plates 27, 28 are then cooled.

The cycle is then restarted. With this operation, the output of thesystem is cyclically brought up to date, with the duration of each cyclebeing on the order of ten minutes. Certain features of the constructionof the mass accumulation sensor system are important and should bementioned. In the illustrated arrangement, the collector plates 27, 28are mounted parallel to the plane of vibration of the tuning forkmembers. This is very important in minimizing the sensitivity to windblasts. It has been found that with the collector plates mountedperpendicular to the plane of vibration, the oscillations are quitesensitive to wind gusts, and small gusts could damp out oscillationentirely. With the plates mounted parallel to the plane of vibration,however, virtually no effect can be discerned by wind blasts from anydirection. The plates 27 and 28 should preferably be oriented in a planeparallel to the ground which allows ice to build up much more evenlythan would be possible if the plates were mounted vertically or at someintermediate angle. The collector plates should have an area largeenough to collect a measurable amount of trace precipitation but smallenough to avoid adding appreciable weight to the tuning fork and toremain rigid. If the plates are too large, undesirable modes ofvibration may be excited, particularly with uneven ice distribution. Ithas been found that an area of about three square inches produces a highdegree of sensitivity without producing undesired vibrational modes orother problems. The plates should be as thin as possible to present verysmall areas in the plane of vibration and to avoid wind effects. Aslight crown or peak in the center of the plates may be provided to aidin ice removal, but the slope should be very slight.

It may be noted even with the blower operative, snow may accumulate onthe plates 27 and 28 to provide an output indication. However, thteindication of the existence of freezing rain or freezing drizzle doesnot depend solely upon the mass accumulation sensor system, since theoutput of the system is combined logically with information gathered bythe other sensors. Accordingly, if there is mass accumulation below 32F. but the other sensors indicate that no rain or drizzle is present,the system output will not indicate such types of precipitation.

IMPACT SENSOR FIGURES 6 and 7 show the construction of the impact sensordevice 31, while FIGURE 6 also shows a block diagram of the impactsensor circuit 21.

The impact sensor system is highly important in that the various formsof precipitation can be categorized by sensing the kinetic energy thatprecipitation particle possesses upon impact. In general, the systemcomprises the device 31 which includes a generally hemispherical plate32, on which precipitation particles may fall, mechanically coupled toan electro-mechanical transducer 14) which produces an electrical signalcorresponding to the kinetic energy of the particle. The output of thetransducer is applied to the circuit 21 which develops output signals atconductors 49, 50, 51 and 52. As indicated above, a signal is developedon conductor 49 in accordance with the amount of precipitation fallingin a certain time interval, the signal being developed when the amountexceeds a certain value. This signal is used in indicating a heavyprecipitation condition.

As also indicated above,.the circuit develops signals on conductors 50,51 and 52 in accordance with the kinetic energy of the precipitationparticles. The signal being developed on the conductor 50 when theenergy is within a certain relatively low range, a signal is developedon conductor 51 when the energy is in an intermediate range, and asignal is developed on the conductor 52 when the energy is above arelatively high value.

The transducer 140 comprises output teminals 141 electrically connectedto a crystal therewithin, the crystal being mechanically coupled to ashaft 142, to generate electrical signals at the output terminals 141 inresponse to angular movement of the shaft 142 about the axis thereof.The shaft 142 has a transverse opening therein to receive one end of apin 143 which is locked in position by means of a thumb screw 144. Thepin 143 forms a lever arm with the plate 32 being supported from thefree ends thereof. The support comprises a vertical strut 145 secured atits lower end to the free end of the pin 143 and secured at its upperend to the center of the plate 32. Four additional struts 146 aresecured at their lower ends to the free end of the pin 143 and extendangularly outwardly and upwardly to points spaced 90 on the plate 32,where they are secured thereto. This arrangement provides a lightweightand yet rigid connection between the plate 32 and the pin 143.

It will be appreciated that as precipitation particles impinge on theplate 32, the shaft 142 is rotated about its 1 1 axis to stress thecrystal of the transducer 140 and gencrate an electrical signal at theterminals 141.

The magnitude of the output voltage is, of course, dependent upon thetype of precipitation. Liquid drops produce smaller output voltages,whereas solid particles produce relatively high voltage outputs, theimpact of a small solid producing an output voltage substantially largerthan that produced by a large drop of water. To eliminate injury to thecrystal from the impact of hailstones or the like, a mechanical stop isprovided to limit movement of the plate 32. In the illustratedarrangement, the housing 33 has a cylindrical portion 147, the upperedge of which is in closely spaced relation to `the lower peripheraledge of the plate 32, to limit the movement thereof.

With regard to the response to liquid drops of various sizes, it isfound that the electrical signal is in the form of a dampened sinusoidalburst having an amplitude approximately proportional to the kineticenergy of the impinging drop. lt has further been found that the kineticenergy of the drop is an exponential function of the mass of the dropalone, there being a distinct impact Velocity for each drop diameter.Thus the voltage output of the transducer depends only on the mass ofthe impinging drop, according to a function which is exponential innature.

To obtain short duration, high voltage output bursts, the resonantfrequency of the mechanical system should be as high as possible, toobtain a quick damp-out of the burst and at the same time to obtain ahigh Q, i.e. a high ratio of energy stored to energy dissipated in onecycle. Thus the system should have the lowest possible mass. However,the largest possible area should be provided to obtain a rate of impacthigh enough to permit measurement even when the rate of precipitation islow. It is found that even in a heavy rain, the rate of impact isrelatively small, on the order of one impact per second per square inch.

With the arrangement as illustrated, a large area is obtained with arelatively small mass and at the same time, the strut arrangementprovides a very rigid structure and eliminates undesired vibrationalmodes. The rigidity is further improved by the use of the plate 32 ofgenerally hernispherical shape. This shape is also important in that itis comparatively insensitive to wind induced errors. If desired, a windscreen may be disposed around the sensing unit.

To prevent the build-up of ice from freezing rain and drizzle, anelectrical heating element 148 is disposed within the cylindricalportion 147 of the housing 33, below the plate 32.

The impact sensor circuit 21 comprises a shaping and filter circuit 149which includes a non-linear shaping network such as to produce an outputsignal Which is a linear function of the mass of the'impacting drop. Asnoted above, the voltage output of the transducer 140 depends only onthe mass of the impinging drop but varies according to a function whichis exponential in nature. The circuit 149 further includes filter meansfor attenuating high frequency components and particularly components inthe frequency ranges generated by aircraft engines, both pistons andjets.

The output of the circuit 149 is fed through an amplitude discriminator150 to a peak reading circuit 151. The amplitude discriminator passesonly those signals above a certain background noise level. The peakreading circuit 151 responds to each pulse signal applied thereto (inresponse to the impact of a particle on the plate 32) to become chargedand to develop at its output a voltage proportional to the amplitude ofthe pulse. The circuit 151 then holds its peak charge until anotherinput pulse causes it to charge or discharge to a new level.

The peak readingrcircuit 151 is periodically reset by a reset timer 152having an input connected to the output of a monostable vibrator 153which is triggered by pulses from the output of the amplitudediscriminator 150. In operation, pulse signals from the amplitudediscriminator trigger the monostable multivibrator 153 to generatepulses used to trigger the reset timer 152. The reset timer remains in atriggered state so long as pulses are applied within short timeintervals from the multivibrator 153. However, when no input pulse isreceived in a certain time interval, the reset timer 152 delivers anoutput pulse to the peak reading circuit 151, whereupon the output ofthe peak reading circuit 151 falls to zero.

|The output of the peak reading circuit 151 is applied to one input of apulse width modulator 154, to control the width or duration of pulsesgenerated thereby. The pulse width modulator 154 has a second inputconnected to the output of a multivibrator 153 for application oftriggering signals thereto. Accordingly, each pulse signal developed atthe output of the amplitude discriminator 1519 triggers the monostablemultivibrator 153 which, in turn, triggers the pulse width modulator,which generates a pulse whose width is determined by the output of thepeak reading circuit 151 which, in turn, is determined by the amplitudeof the pulse signal. The output of the pulse width modulator 154 isapplied to an averaging circuit 155 which develops an outputproportional to the integrated or average value of the square wavepulses developed by the pulse width modulator 154. The output of theaveraging circuit 155 is thus proportional to the rate of precipitationand, although not used in the system 1l), the signal therefrom may beusable directly or in other systems. For this purpose, an outputterminal 156 is connected to the output of the averaging circuit 155.

The output of the averaging circuit 155 is also applied through anamplitude discriminator 157 to a squaring circuit 158 having its outputconnected to the conductor 49. When the precipitation rate reaches apredetermined relatively high value, the amplitude discriminator 157applies a signal to the squaring circuit 158 to develop a control signalat the conductor 49, for use in the system 10 as above described.

The output of the peak reading circuit 151 is also applied through threeamplitude discriminators 159, and 161 to squaring circuits 162, 163 and164. Amplitude discriminator 159 is operative when the kinetic energy ofthe precipitation particles exceeds a certain low value, discriminator160 is operative when the kinetic energy 0f the particles exceeds acertain medium value, and discriminator 161 is operative when thekinetic energy of the particles exceeds a certain relativelly highvalue. The output conductor 50 is connected to the output of an ANDcircuit 155 having a first input connected to the output of the squaringcircuit 162, a second input connected through an inverter 166 to theoutput of the squaring circuit 163, and a third input connected throughan inverter 167 to the output of the squaring circuit 164. With thisarrangement, an output signal is developed on the conductor 50 whenthere is concurrently an output from the low amplitude discriminator159, and no outputs from the medium and high discriminators 161) and161.

The output conductor 51 is connected to the output of an AND circuit 163having a tirst input connected to the output of the squaring circuit 163and a second input connected through the inver-ter 167 to the output ofthe squaring circuit 164. Thus an output is developed on the conductor51 when there is concurrently an output from the medium amplitudediscriminator 160 and no output from the high amplitude discriminator161.

Conductor 52 is connected directly to the output of the squaring circuit164, to have arl output signal developed thereon when the high amplitudediscriminator 161 is operative.

REBOUND SENSOR FIGURE 8 shows the construction of the rebound sensingapparatus 34 and also shows a block diagram of the 'rebound sensorcircuit 22. As noted above, the apparatus 34 comprises inclined plates35, 36, 37 and 38 on which precipitation falls and from which it mayrebound against lower surface portions of an impact sensing structure39, when the precipitation is of solid form, such as sleet or hail. Thecircuit 22 functions to produce an output signal on conductor 53 whenthere is rebound, and to produce no output signal when there is norebound.

The impact sensing structure 39 comprises a sensing member 170 againstwhich the rebounding particles may impinge. Preferably, the member 170has four inclined surfaces generally parallel to the surfaces of theplates 35-38, so as to provide an inverted pyramidal shape. To developan electrical signal in response to movement of the member 170, anelectro-mechanical transducer is provided in the form of fourpiezoelectric crystals 171, the lower faces of the crystals 171 beingsecured to the upper side of the member 170. The upper faces of thecrystals 171 are secured to a mounting plate 172 which, in turn, issecured within a support block 173. A cushion 174 is provided betweenthe lower face of the block 173 adjacent its peripheral edge and theupper face of the member 170.

It will be appreciated that when a solid precipitation particle strikesone of the plates 35-38 with sufficient velocity to rebound against thelower surface of the member 170, the crystals 171 will be compressed togenerate an electrical voltage between the upper and lower facesthereof. To transmit the electrical signal to the circuit 22, the lowerfaces are electrically connected to the center conductor of a coaxial orshielded transmission line 17S while the upper faces of the crystals 171are electrically connected to the block 173 which, in turn, is connectedelectrically to the outer or shield conductor of the line 175.

The support block 173 is resiliently suspended from a plate 176 by meansof stud bolts 177, nuts 178 and resilient washers 179, disposed betweenthe nut 178 and the upper surface of the plate 176, and also disposedbetween the lower surface of the plate 176 and the upper surface of theblock 173.

The plate 176 is secured at its opposite ends to the lower surfaces ofthe central portions of a pair of horizontal support bars 180, supportedby means of uprights 181 from a base plate 182. The structure 39 mayfurther include a top cover plate 183 and four shield plates 184disposed adjacent the four sides of the block 173, preferably with thelower edges of the shield plates 184 being disposed slightly below theperipheral edges of the member 170. The rebound plates 35-38 aresupported from the baseplate 182 through upright walls 185 and spacermembers 186 between the upper inside edge portions of the walls 185 andupper edge portions of the plates 35- 38. The adjoining edges of theplates 35-38 are preferably secured together as by welding.

To prevent formation of ice on the plates 35-38, electrical heaters 187are insalled on the lower surfaces thereof. A pair of electrical heaters188 are preferably installed in the block 173, to prevent formation ofice on the member 17 0 and associated structure. The heaters 187 and 188may be connected to a suitable source of electricity, preferably throughthermostat means operative when the temperature drops below a certainvalue.

The circuit 22 comprises a peak reading circuit 189 having an inputconnected through an amplitude discriminator 190 and a noise filter 191to the output of the crystal array 171, and having an output connectedthrough an amplitude discriminator 192 and a squaring circuit 193 to theoutput conductor 53 when the peak value of an input signal exceeds acertain value, as determined by the amplitude discriminator 192. Theamplitude discriminator 190, in the input circuit of the peak readingcircuit 189, functions to eliminate background noise signals, while thenoise iilter 191 may attenuate high frequency signals, such as may becaused by jet or piston-type aircraft.

The peak reading circuit 151 responds to each pulse signal appliedthereto to become charged and to develop at its output a voltageproportional to the amplitude of the pulse. The circuit 189 then holdsits peak charge until another input pulse causes it to charge ordischarge to a new level. To prevent the circuit from maintaining anoutput signal when rebounds are no longer detected, the circuit 189 isperiodically reset by a reset timer 194 having an input connectedthrough a monostable multivibrator 195 to the output of the sensingapparatus 34. In operation, pulse signals from the sensing apparatustrigger the monostable multivirbator 195 to generate pulses whichtrigger the reset timer 194. The reset timer 194 remains in a triggeredstate so long as pulses are applied within short time intervals from themultivibrator 195. However, when no input pulse is received in a certaintime interval (which may be on the order of 10 seconds), the reset timer194 delivers an output pulse to the peak reading circuit 189, whereuponthe output of the peak reading circuit 189 falls to Zero.

PHOTO-OPTICAL SENSOR FGURE 9 shows the construction of the photo-opticalsensing apparatus 40 and also shows a block diagram of the photo-opticalsensor circuit 23. As described above, the precipitation falls through acentral portion 41 of the apparatus 40, between portions 42 and 43thereof. Portion 42 houses a light source 197 which projects lightthrough the portion 41 toward the portion 43, and also houses aphotoelectric detector, generally designated by reference numeral 198,which develops electrical signals Vin response to the reflection oflight from particles of precipitation falling through the portion 41.The output of the detector 198 is applied to the sensor circuit 23 whichdevelops output signals on the conductors 56, 57 and 58, a signal beingdeveloped on conductor 56 when the reflection of light is in arelatively low range, a signal being developed on conductor 57 when thereilection of light is in a certain intermediate range, and a signalbeing developed on conductor 58 when the reflection of light exceeds arelatively high value.

The photoelectric detector 198 comprises a parabolic mirror mountedwithin a housing 200, and a suitable photoelectric cell 201, preferablya photo-diode, supported at the focal point of the parabolic mirror bymeans of a pair of supports 202 extending from the housing 200. Thesupports 202 may be hollow to carry connection wires which extend fromthe housing 200 to the circuit 23 through a suitable cable 203.

The photoelectric detector 198 and the light source 197 are supportedwithin a housing which includes a bottom wall 204, a top wall 205, apair of side walls 206 and an end wall 207. A wall 208 extendsdownwardly from the forward edge of the top wall 205 part way toward thebottom wall 204, while a plate 209, having a window 210 therein, extendsfrom the lower inside edge of the plate 288 downwardly and inwardly, andthen downwardly and outwardly to the forward edge of the bottom wall284. A plate 211 is disposed between the side walls 206 over thephotoelectric detector 198, and another plate 212 is disposed betweenthe side walls 206, between the light source 197 and the photoelectricdetector 198. The surfaces of the plates 209, 211 and 212, especiallythose which face the photoelectric detector are preferably dull,light-absorbant surfaces, to minimize transmission of undesired light tothe photoelectric detector 198.

The portion 43 of the apparatus 40 functions to provide a darkbackground, to obtain maximum contrast. Portion 43 comprises a top wall213, a bottom wall 214, an end wall 216 and an end wall 217 having anopening 218 therein.

To provide the dark background, a plate 219 having a dull surface ismounted within the portion 43 in alignment with the axis of thephotoelectric detector 198 and at an angle such that any light reflectedtherefrom toward the photoelectric detector 198 comes from the directionof another plate 220. Plate 220 also has a dull surface and is locatedat an angle such that only light coming from directly above could bereflected to the plate 219 and thence to the photoelectric detector 198.The lower surface of the top wall 213, as well as the other internalsurfaces of the housing should, of course, have dull surfaces. With thisarrangement, the reflection of light to the photoelectric detector 198is minimized.

It has been found that reection of light from particles of precipitationfalling through the portion 41 of the apparatus is increased byproviding a mirror 221 disposed above the plate 219 and in alignmentwith the light source 197, at such an angle as to cause reflection oflight from the upper surfaces of precipitation particles to thephotoelectric detector 198.

It is important that condensation and freezing of moisture on thevarious surfaces be minimized, particularly on the plate 219, since suchcondensation and freezing can greatly increase the coetcient ofreflectivity. For this reason, an electrical heater 219a is preferablyinstalled on the back side of the plate 219.

The circuit 23 comprises an amplifier 222 having an input connected tothe output of the photoelectric detector 198 and having an outputconnected to three amplitude discriminators 223, 224 and 225. Each ofthe amplitude discriminators 223-225 is in the form of a switchingcircuit whose output remains in one state until the input level reachessome preset value at which time the output switches abruptly to adifferent state. The output then remains in the second state until theinput returns to a value just below the preset level, and the outputthen switches back to the first state. Amplitude discriminator 223triggers at a relatively low input pulse amplitude, discriminator 224triggers at an intermediate input pulse amplitude and discriminator 225triggers only at a relatively high input pulse amplitude.

The outputs of the amplitude discriminators 223, 224 and 225 arerespectively applied to flip-flop circuits 226, 227 and 228 and are alsoapplied to first inputs of three OR circuits 229, 23) and 231 havingsecond inputs connected to the outputs of the ilip-flop circuits 226,227 and 228. The output of the OR circuit 229 is connected to one inputof an AND circuit 232 having its output connected to the conductor 56,having a second input connected through an inverter 233 to the output ofthe OR circuit 230 and having a third input connected to the output ofthe OR circuit 231 through an inverter 234. The output of the OR circuit230 is connected to one input of an AND circuit 235 having its outputconnected to the conductor 57 and having a second input connectedthrough the inverter 234 to the output of the OR circuit 231. The outputof the OR circuit 231 is directly applied to the conductor 58.

The Hip-flop circuits 226, 227 and 228 are arranged to be reset atcertain times by pulses from reset timers 236, 237 and 238 having inputsconnected to the outputs of the amplitude discriminators 223, 224 and225. Each of the reset timers 236438 is triggered in response to a pulsereceived from the respective one of the amplitude discriminators 223-225and remains in a triggered state so long as pulses are applied withinshort time intervals. However, when no pulse is applied within a certaintime interval, each reset timer delivers an output pulse to therespective ip-ilop circuit, to reset the flip-flop circuit.

In operation, if only the low amplitude discriminator 223 is triggered,a signal is applied to the OR circuit 229 and also a set signal isapplied to the ip-op circuit 226, to apply a second signal to the ORcircuit 229. The signal from the output of the OR circuit 229 is appliedto one input of the AND circuit 232 and if no signals are developed fromthe OR circuits 230 and 231, the AND circuit 232 will be energizedthrough the inverters 233 and 234, developing an output signal on theconductor 56.

When the pulse signal is developed at the output of 16 the low amplitudediscriminator 223, the reset timer 236 is triggered, and it will remaintriggered if additional pulses are received within a certainpredetermined time interval. However, if no input pulses are appliedwithin the predetermined time interval, the reset timer applies anoutput pulse to the hip-flop circuit 226, to operate the flip-flopcircuit 226 to its reset condition. The OR circuit 229 will no longerfunction to apply a signal to the AND Y circuit 232, and the signaloutput on the conductor 56 will disappear.

The hip-flop circuits 227 and 228 are operated in a similar way throughsignals from the medium and high amplitude discriminators 224 and 225,and by signals from the reset timers 237 and 238. If the amplitudediscriminator 224 delivers a pulse, the flip-flop 227 will be triggeredto its set condition, to apply a signal through the OR circuit 238 andthe AND circuit 235 to the conductor 57, assuming that the ip-tlop 228,associated with the high amplitude discriminator 226, is in its resetcondition. At the same time, no signal is applied to the middle input ofthe AND circuit 232, by operation of the inverter 233, and thus therewill be no signal on the conductor 56 and only the conductor 57 will beenergized.

If the high amplitude discriminator 225 delivers an output pulse, theconductor 58 is energized directly from the hip-flop 228. Through theinverters 233 and 234 operative on the AND circuits 232 and 235, therewill be no output signal on the conductors 56 and 57. Hence only theconductor 58 will have an output signal thereon.

AND CIRCUIT The AND circuits 66-75, 77, 78, 123, 265, 168, 232 and 235referred to above may be of any desired construction, a preferredconstruction being illustrated in FIGURE l0. Referring thereto, an ANDcircuit is illustrated having four input terminals 241, 242, 243 and 244in addition to a common grounded terminal 245, and having a pair ofoutput terminals 246 and 247, terminal 247 being grounded.

The circuit is arranged to produce a positive output signal at theterminal 246 when positive input signals are concurrently applied to allof the input terminals 241-244. For this purpose, input terminals241-244 are respectively connected to a circuit point 248 through diodes249-252, circuit point 248 being connected through a resistor 253 to aterminal 254 arranged for connection to the positive terminal of a D.C.source having its negative terminal connected to ground, terminal 254being preferably at plus 25 volts relative to ground. With thisarrangement, the circuit point 248 is at a potential determined by theleast positive one of the input terminals 241-244. If any one of theinput terminals 241-244 is placed at ground potential the circuit point248 will be at approximately ground potential. Thus a positive potentialis developed at circuit point 248 only if all of the terminals 241-244are concurrently at positive potentials. Circuit point 248 is connectedthrough a diode 255 to the output terminal 246 which is connectedthrough a resistor 256 to a terminal 257 arranged for connection to thenegative terminal of a D.C. power supply, terminal 257 being preferablyat minus 25 volts relative to ground. The diode 255 normally conducts toplace terminal 246 at a potential approximately equal to that of thecircuit point 248. However, diode 255 serves as an isolation device, topermit the output terminal 246 to be at a positive potential, throughits interconnection with other circuits, even when one of the inputterminals 241-244 is at ground potential. It will be appreciated, ofcourse, that the AND circuit of FIGURE 10 may have any desired number ofinputs.

OR CIRCUIT The OR circuits 76, 79 and 229-231 may have any desiredconstruction, a preferred circuit being illustrated in FIGURE ll.Referring thereto, an OR circuit is illustrated having three inputterminals 258, 259 and 260, in addition to a common grounded inputterminal 261, and having output terminals 262 and 263, terminal 263being grounded. The circuit is arranged to produce a positive outputsignal at terminal 262 when a positive signal is applied to any one ormore of the input terminals 258-260. For this purpose, input terminals258- 260 are connected through diodes 264-266 to a circuit point 267which is connected through a resistor 268 to a terminal 269 arranged forconnection to the negative terminal of a D.C. power supply having itspositive terminal connected to ground, terminal 269 being preferably atminus 25 volts relative to ground.

In operation, when a positive voltage is applied to any of the inputterminals 258-260, the corresponding one of the diodes 264-266 willconduct to place circuit point 267 and the output terminal 262 at apositive potential closely approaching that of the applied input signal.However, if all of the input terminals 258-260 are at ground potentials,the output terminal 262 will likewise be at ground potential.

INVERTER AMPLIFIER CIRCUIT The circuits 48, 55, 61, 64, 166, 167, 233and 234 as above described, may have any desired construction, apreferred inverter circuit being illustrated in FIGURE l2. Referringthereto, the circuit has a pair of input terminals 270 and 271 and apair of output terminals 272 and 273, terminals 271 and 273 beinggrounded. The circuit is arranged to produce a positive output signal atoutput terminal 272 when the input terminal 270 is at ground potential,and to produce substantially no output signal at the output terminal 272when the input terminal 270 is at a positive potential.

In particular, terminal 270 is connected through the parallelcombination of a resistor 274 and a capacitor 275 to the base of atransistor 276, the base being connected through a biasing resistor 277to a terminal 278 arranged for connection to a negative terminal of aD.C. power supply having a positive terminal connected to ground. Theemitter of the transistor 276 is connected to ground while the collectorthereof is connected to the output terminal 272 and also to a resistor279 to a terminal 280, arranged for connection to the positive terminalof a D.C. power supply having its negative terminal connected to ground.

In operation, if the input terminal 270 is at ground potential, there issubstantially no conduction through the transistor 276, and the outputterminal 272 is at a positive potential approaching that of the terminal280. When a positive potential is applied to the terminal 270, thetransistor 276 is rendered conductive, and the output terminal 272 isplaced at a low potential, approaching ground potential. Resistor 274serves fto limit the baseemitter current of the transistor 276, whilethe capacitor 275 improves the speed of response of the circuit.

F LIP-FLOP CIRCUIT The ip-op circuits 109 and 124 of FIGURE 2 and theflip-flop circuits 226-228 of FIGURE 9 may likewise have constructionssuch as are well known in the art, a preferred flip-flop circuit beingillustrated in FIG- URE 13.

Referring to FIGURE 13, a pair of output terminals 281 and 282 areconnected to the collectors of a pair of transistors 283 and 284 whichare so connected that one is conductive while the other is cut H. Theemitters of the transistors are connected to ground through resistors285 and 286 in parallel with capacitors 287 and 288. The collectors areconnected through resistors 289 and 290 to a terminal 291 arranged forconnection to the positive terminal of a D.C. power supply. The baseelectrodes of the transistors 283 and 284 are connected to a terminal292, arranged for connection to the negative terminal of a power supplywhose positive terminal is con- 18 nected to ground, through biasingresistors 293 and 294.

To render one transistor conductive while the other is cut olf,cross-connections are provided between the base and collector electrodesthereof. In particular, the parallel combination of a resistor 295 and acapacitor 296 is connected between the collector of transistor 283 andthe base of transistor 284 while the parallel combination of a resistor297 and a capacitor 298 is connected between the collector of thetransistor 284 and the base of the transistor 283.

With such cross-connections and with the bias obtained throughconnection of the base electrodes to the negative terminal 292 throughthe resistors 293 and 294, one transistor will be cut off while theother is conductive and the circuit will remain stably in such acondition, until a switching signal is applied to one of the conductors,whereupon the reverse action takes place.

The switching signals may be applied in various ways. In particular, anA.C. switching signal may be applied to one or the other of a pair ofinput terminals 299 and 300. Terminal 299 is connected through acapacitor 301 to a circuit point 302 which is connected to groundthrough a resistor 303, circuit point 302 being connected to the baseelectrode of the transistor 283 through a diode 304. Similarly, terminal300 is connected through a capacitor 305 to a circuit point 306connected to ground through a resistor 307 and connected to the baseelectrode of transistor 284 through a diode 308.

If it is assumed that the transistor 283 is cut off while the transistor284 is conducting, a positive-going signal may be applied to theterminal 299 to develop a positive pulse at the circuit point 302 whichis applied through the diode 304 to the base of the transistor 283.Transistor 283 then starts to conduct whereupon the potential of itscollector moves in a negative direction, causing the potential of thebase of the transistor 284 to move in a negative direction to decreaseconduction through the transistor 284. As conductance through thetransistor 284 is decreased, the potential of its collector moves in apositive direction thus applying a positive signal to the base of thetransistor 283 through the capacitor 298. As a result, the circuit israpidly switched to` a condition in which the transistor 283 conductsheavily while the transistor 284 is cut off. A positive-going signal maythen be applied to the input terminal 300 whereupon the circuit will beswitched back to its initial condition.

Control signals may also be applied to either of a pair of terminals 309and 310, connected through diodes 311 and 312 to the base electrodes ofthe transistors 283 and 284. When a negative potential is appliedthrough the terminal 309, the transistor 283 can be cut oi, or can bekept from becoming conductive. Similarly, the negative potential may beapplied to the terminal 310 to cut off the transistor 284 or to preventit from becoming conductive.

Although not so used in the circuits thus far described, the flip-flopcircuit of FIGURE 13 may also be used as a binary counter device. Forthis purpose, pulse signals may be applied to an input terminal 313connected through capacitors 314 and 315 to circuit points 316 and 317which are connected to ground through resistors 318 and 319 and whichare connected to the base electrodes of the transistors 283 and 284through diodes 320 and 321. When a positive pulse is applied to theterminal 313 one or the other of the transistors 283 or 284 will berendered conductive while the other will become cut oi. The nextpositive pulse applied to the terminal 313 will cause the circuit toreverse to its previous condition.

When the hip-flop of FIGURE 13 is used as the circuit 109 of FIGURE 2,the output of the squaring circuit 103 may be connected to the terminal299 while the output of the squaring circuit 104 may be connected to theterminal 300. The output terminal 282 may then be connected to theaveraging circuit 110. In response to the leading edges of the signalsfrom the squaring` circuit 103,

i.e. the leading edges of the signal 105 shown in FIGURE 4v or thesignal 107 shown in FIGURE 5, the transistor 283 may be shifted from anon-conductive state to a conductive state while the transistor 284 isshifted from a conductive state to a non-conductive state. A positivesignal is then developed at the output terminal 282. It is noted thatthe capacitor 301 together with the resistor 303 should have acomparatively short time constant, to provide a differentiating action.

Similarly, when the leading edges of the signals from the squaringcircuit 104, i.e. Vthe leading edges of the signal 106 of FIGURE 4 o1'the signal 108 of FIGURE 5, are applied to the terminal 300, thetransistor 284 is shifted from anon-conductive state to a conductivestate while the transistor 283 is shifted from a conductive'state to anon-conductive state. When transistor 284 is conductive, the potentialof the output' terminal 282 is reduced to a value approaching groundpotential. Accordingly, waveforms are developed such as indicated by thereference numerals 111 and 113 in FIGURES 4 and 5.

With the flip-flop circuit of FIGURE 13 usedv as the circuit 124 ofFIGURE 2, the output of the AND circuit 123 may be connected to theterminal 299, while the terminal 136 of the timer and control circuit127 may be connected to the terminal 309, terminal 282 being connectedto the output conductor 46. In'operation, a negative signal may beapplied from the terminal 136 to the terminal 309 to cause theytransistor 283 to be non-conductive and toprevent conduction thereof. Atcertain times, as described above, a signal may be applied from theterminal 136 to render the flip-flop circuit 124 operative. This signalmay be a positive signal, applied to the terminal 309. If, then, apositive signal is applied from the AND circuit 123 to the terminal 299,the transistor 283 is shifted from a non-conductive state to aconductive state while the transistor 284 is shifted from a'conductivestate to a non-conductive state thereby developing a'positive voltage atthe output terminal 282, connected to the conductor 46.

With the ip-op circuit4 of FIGURE 13 used for the circuits 226, 227 and228 of FIGURE 9, the outputs of the amplitude discriminators 223-225 maybe connected to the terminal 299. The terminal 300 may then be connectedto the output of the associated one of the reset timers 236-238. Theoutput may be connected then to the output terminal 282. Undersuch'circumstances, when a positive pulse is applied from the amplitudediscriminator to the terminal 299, the transistor 283 is shifted from anon-conductive state to a conductive state while the transistor 284 isshifted from a conductive state to a non-conductive state, therebydeveloping a positive signal at the output terminal 282. When a pulse isapplied from the reset timer to the terminal'300, the circuit is thenshifted back to its initial condition with the transistor 284 conductiveand with the transistor 283 cut olf.

MULTIVIBRATOR CIRCUIT conductive state to a conductive state while'thetransistor 324 is shifed from a conductive state to a non-conductivestate. A positive signal is then developedat the collector of thetransistor 324, which is connected to an output terminal 327. Thepositive signal is also applied through' a resistor 328 to the emitterofthe unijunction transistor 325, a capacitor 329 being connectedbetween the emitter of the transistor S-and ground. When the positivesig- Whena positive pulse is applied to an inputV terminal 326, thetransistor 323 is shifted from a non-V nal isapplied from the collectorof the transistor 324, the capacitor 329 starts -to charge up and aftera certain time interval, the voltage across the capacitor 329 exceedsthe breakdown voltage between the emitter of the unijunction transistor325 and the first base 330 thereof, which is connected to a resistor 331to ground. A positive pulse is then developed across the resistor 331which is applied through'a capacitor 332 to a circuit point 333connected through a diode 334 to the base of the transistor 324, circuitpoint 333 being connected through a resistor 335 to ground. Thus apositive pulse is applied to the basevof the transistor 324. This causesthe transistor 324 to shift from a non-conductive state to a conductivestate, while the transistor 323 is shifted from a conductive state to anon-conductive state. Thus the circuit is in its initial condition. Theoutput voltage, at terminal 327, drops to a low value.

Accordingly, a positive pulse is developed at the output terminal 327 ofa certain duration, determined primarily by the time constant of thecircuit including resistor 328 and capacitor 329.

As is conventional, the second base of the unijunction transistor 325 isconnected through a resistor 336 to a terminal 337 arranged forconnection to the positive terminal of a D.C. power supply having itsnegative terminal connected to ground.

It will be recognized that the transistors 323 and 324 are connected ina manner similar to the connection of transistors 283 and 284 in theflip-flop circuit of FIGURE 13. The collectors are connected throughresistors 339 and 340 to the power supply terminal 337. The emitters areconnected to ground through resistors 341 and 342 and capacitors 343 and344. The collector of transistor 323 is connected to the base of thetransistor 324 through the parallel combination of a capacitor 345 and aresistor 346, while the collector of the transistor 324 is connected tothe base of the transistor 323 through the parallel combination of acapacitor 347 and a resistor 348. The bases of the two transistors areconnected to resistors 349 and 350 to a terminal 351 arranged forconnection to the negative terminal of a D.C. power supply having itspositive terminal connected to ground.

To apply the switching pulse from the input terminal 326, it isconnected through a capacitor 352 to a circuit point 353 connected toground through a resistor 354 and connected to the base of thetransistor 323 to a diode 355. Preferably, the capacitor 352 togetherwith the resistor 354 should have a comparatively low time constant, toprovide a differentiating action.

In the event that a negative output pulse is desired, rather than apositive output pulse, an ouput terminal 356 may be used, connected tothe collector of the transistor 323.

RESET TIMER CIRCUIT The reset timer Vcircuits 152, 194 and 2364238,discussed above, may preferably have a construction such as illustratedin FIGURE 15. Referring thereto, the circuit is arranged to be triggeredby a pulse applied to an tinput terminal 368 and to remain triggered solong as additional pulses are applied, with the time intervals betweenpulses not exceeding a certain time interval. However, when no inputpulse is applied to the terminal 360 within a fcertain time interval,the circuit delivers an output pulse at an output terminal 361.

The input terminal 360 is connected to the input of a pulse ampliier 362having output terminals 363 and 364. When the pulse is applied to theinput of the pulse amplifier 362, an output pulse is immediatelydeveloped at terminal 363 while a pulse is developed at the terminal 364after a certain delay time.

Terminals 363 and 364 are connected to input terminals of AND circuits365 and 366 having outputs connected to monostable multivibrators 367and 368, which may be constructed in the manner as illustrated in FIG-URE 14, and as described above. The outputs of the multivibrators 367and 368 are connected to inputs of the AND circuits 365 and 366 and arealso connected through pulse amplifiers 369 and 378 to inputs of anotherpair of AND circuits 371 and 372. In addition, the output of themultivibrator 367 is connected to another input of the AND circuit 372,while the output of the multivibrator circuit 368 to another input ofthe AND circuit 371 and also through an inverter 373 to a third input ofthe AND circuit 365. The outputs of the AND circuits 371 and 372 areconnected to inputs of an OR circuit 374, having an output connected tothe output terminal 361.

In operation, before any input pulse is applied to the input terminal360, the outputs of both multivibrators 367 and 368 are postive.Assuming that the multivibrator circuit of FIGURE 14 is used, the outputmay be taken at the output terminal 356, the transistor 323 beingnormally non-conductive.

When a pulse is applied to the pulse amplifier 362, a pulse isimmediately developed at the output terminal 363 and is applied to theAND circuit 365. However, no pulse is transmitted to the multivibrator367, since the AND circuit 365 is inhibited by a negative signal appliedfrom the inverter 373. After a certain delay time, a pulse is appliedfrom the terminal 364 to the AND circuit 366 which applies the pulse tthe input of the multivibrator 368. When the multivibrator 368 istriggered, an inhibiting signal is applied to the AND circuit 366 andthrough the inverter 373, an enabling signal is applied to the ANDcircuit 365.

If it is assumed that no further input pulses are applied to theterminal 360, the monostable multivibrator 368 will, after a certaintime interval, switch back to its initial condition. It then applies apulse to the pulse amplifier 37) which applies a pulse through the ANDcircuit 372 and the OR circuit 374 through the output terminal 361. TheAND circuit 372 is enabled,` since the multivibrator 367 is in itsinitial condition with a positive output therefrom. Accordingly, apositive output pulse is developed at the terminal 361 if no furtherinput pulse is applied within the time interval of operation of themultivibrator 368.

If, however, a second input pulse is applied before the multivibrator368 times out, it will develop an output at the terminal 363 of thepulse amplifier 362, which is applied through the AND circuit 365 to themultivibrator 367 to trigger the same. The multivibrator 368 maysubsequently time out to apply a pulse to the pulse amplifier 370.However, the pulse will not be applied through the AND circuit 372,since the AND circuit 372 will be inhibited through a signal appliedfrom the multivibrator 367. If no further input pulse is applied, themultivibrator 367 will time out after a certain time interval and willdeliver a pulse to the pulse amplifier 369 and through the AND circuit371 and the OR circuit 374 to the output terminal 361. The AND circuit371 will be enabled, since a positive signal is at this time appliedfrom the output of the multivibrator 368.

However, if it is assumed that a third pulse is applied before themultivibrator 367 times out, it will develop a delayed pulse at theterminal 364 which will trigger the multivibrator 368. Then when themultivibrator 367 times out, no pulse will be applied to the outputterminal 361, since the AND circuit 371 will be disabled through thesignal applied from the triggered multivibrator 368. If no further pulseis applied, the multivibrator 368 may time out to deliver a pulse to theoutput terminal 361 in the manner as above described. However, ifanother pulse is received, the multivibrator 367 may be triggered.

Accordingly, one or the other of the multivibrators 367 and 368 will betriggered so long as input pulses are received with a time delaytherebetween not exceeding a certain time interval, and no output pulsewill be developed at the terminal 361. However, when no input pulse isreceived in a certain time interval, an output pulse will be developedat the terminal 361.

2.2 AMPLITUDE DISCRIMINATOR-SQUARING CIRCUIT The circuit of FIGURE 16may preferably be used for the various amplitude discriminator andsquaring circuits referred to above. This circuit comprises a pair oftransistors 377 and 378 having emitters connected together and through acommon resistor 379 to ground and having collectors connected to apositive power supply terminal 388 through resistors 381 and 382.

The base of the transistor 378 is connected to ground through a resistor383 and is connected through the parallel combination of a resistor 384and a capacitor 385 to the collector of the transistor 377, thepotential of the base of the transistor 378 being thereby controlled bythe potential of the collector of the transistor 377. The collector ofthe transistor 378 is connected to an output terminal 386, while thebase of the transistor 377 is connected through a resistor 387 to aninput terminal 388, input terminal 388 being connected through a biasingresistor 389 to a terminal 390 arranged for connection to a suitablesource of biasing potential.

In operation, it may be assumed that the potential of the bias terminal398 is suiiciently negative to prevent conduction through the transistor377. The collector thereof will then be at a highly positive potentialand through the resistor 384, the base of the transistor 378 will bebiased positively and there will be a high current flow through thetransistor 378. The output terminal 386 will then be at a low potential,approaching ground potential.

When the input signal applied to the terminal 388 has a potentialexceeding a certain value, the circuit will be rapidly switched to acondition wherein the transistor 377 will conduct heavily while thetransistor 378 will be cut oit, thus producing a highly positivepotential at the output terminal 386. Two features of the circuitcontribute to the rapid switching operation. First, as the transistor377 starts to conduct, the current ow through the resistor 379 will beincreased, to decrease the base-to-emitter voltage of the transistor378. Secondly, as the transistor 377 starts to conduct, the potential ofthe base of the transistor 378 will be dropped through the capacitor385. In any event, as a result of a comparatively slight elevation inthe potential of the input terminal 388, the circuit is shifted from acondition in which the transistor 378 conducts heavily to a condtion inwhich it is cut olf, to produce a highly positive output voltage.

lVhen the input voltage is then dropped below a certain value, thereverse action takes place and the circuit will be rapidly switched to acondition in which the transistor 378 again conducts heavily while thetransistor 377 will be cut olf. The connection of the emitters to groundthrough a common resistor 379 contributes to the rapid switchingoperation.

It will thus be appreciated that the circuit of FIGURE 16 provides asquaring action and is also usable as an amplitude discriminator. Thepotential of the terminal 398 may be adjusted by any suitable means, tocontrol the point at which the switching action takes place.

PULSE AMPLIFIER FIGURE 17 illustrates a preferred circuit for a pulseamplifier which may be used as the amplifier 362 and amplifiers 369 and370 in the circuit of the reset timer, as illustrated in FIGURE l5.

The pulse amplifier circuit of FIGURE 17 comprises a transistor 391having an emitter connected to ground through the parallel combinationof a resistor 392 and a capacitor 393 and having a collector connectedthrough a primary winding 394 to a positive power supply terminal 396.The base of the transistor 391 is connected through a resistor 397 and adiode 398 to a circuit point 399 which is connected to ground through aresistor 400.

The hase is also connected through a biasing resistor 401 to a negativepower supply terminal 402.

As a result, the transistor 391 is normally nonconductive or conducts acomparatively small amount of current. Conduction thereof may beincreased, however, by application of a pulse to the base through acapacitor 403 connected to a pulse input terminal 404. When a positivepulse is applied to the terminal 404, there is a build-up of currentiiow through the inductive impedance formed by the transformer primary394. A voltage is then developed across a secondary winding 405 of thetransformer 395, which is applied through a capacitor 406 to the circuitpoint 399, and thence through resistor 397 and diode 398 to the base ofthe transistor 391. It should be noted that the lower terminal of thewinding 405 is grounded.

As a result of the application of this feedback voltage, the build-up ofcurrent through the transistor 391 is increased, thus further increasingthe voltage developed across the secondary winding 405. As a result, acomparatively high voltage is rapidly developed across the winding 405.This voltage is applied through a diode 407 in parallel with a resistor408 to the base of a transistor 409 having its emitter connected to anoutput terminal 410 and also connected through a resistor 411 to thepositive power supply terminal 396. The base of the transistor 409 isadditionally connected through a biasing resistor 412 to the negativepower supply terminal 402. The collector of the transistor 409 isgrounded.

The transistor 409 operates as an emitter-follower. Normally, itconducts heavily and the potential of the output terminal 410 isapproximately at ground potential. When the positive pulse is appliedfrom the transformer secondary winding 405 through diode 407 andresistor 403 to the base of the transistor 409, its conduction isdecreased to zero or a low value, producing a large positive outputvoltage at the output terminal 410.

To produce a delayed output' signal, a second winding 413Y is providedon the transformer 395. The upper terminal of the winding 413 isconnected to ground while the lower terminal thereof is connectedthrough a diode 414 to another output terminal 415 which is connected toground through a resistor 416.

In operation, the current through the transformer primary 394 builds upvery rapidly in response to application of an input pulse, in a manneras above described, and the 1ield in the core of the transformerincreases correspondingly. After the current and eld reach peak values,they then decrease rather rapidly producing a voltage of the oppositepolarity in the winding 405 and also in the winding 413. This oppositepolarity voltage in the winding 413 is applied through the diode 414 tothe output terminal 415. Accordingly, a slightly delayed output pulse isobtained.

It will be appreciated that when the circuit of FIGURE 17 is used forthe pulse amplifier 362 of FIGURE l5, the input terminal 360 isconnected to the input terminal 404, the output terminal 363 correspondsto the output terminal 410, and the output terminal 364 corresponds tothe output terminal 415.

In the reset timer circuit of FIGURE as above described, the pulseamplifiers 309 and 370 respond to a sudden change in the outputs of themultivibrators 367 and 368, from a low value to a highly positive value,occurring when the multivibrators time out. A somewhat different inputcircuit is desirable for the pulse amplifiers 369 and 370. Inparticular, an input terminal 417 may be provided in the circuit ofFIGURE 17 connected through a diode 418 and a resistor 419 to the pulseinput terminal 404 which is connected through the capacitor 403 to thebase of the transistor 391.

It is believed that the construction of the remaining circuits is wellknown in the art and, accordingly, they are not illustrated anddescribed in detail. The peak reading circuit 151 of FIGURE 6 maycomprise a capacitor connected to the output of a transmission gatecircuit, various forms of which are well known in the art, in which theoutput signal corresponds to the input signal during a selected timeinterval which is controlled by a gating signal, and in which the outputis zero and the output impedance is high except during application ofthe gating signal. The pulse whose peak amplitude is to he registeredand may be applied to the input of the transmission gate circuit andrnay also act as a gating signal so that the capacitor is charged ordischarged during the applied pulse, to a value proportional to the peakamplitude of the pulse.

The pulse width modulator 154 may comprise a monostable multivibratortriggered by signals from the multivibrator 1753 with the signal fromthe peak reading circuit 151 being applied to the timing circuit of themonostable multivibrator to control the duration of the pulses generatedthereby. For example, the multivibrator circuit of FIGURE 14 may be usedwith the input terminal 326 being connected to the output of themultivibrator 153. The voltage output of the peak reading circuit 151may then be applied in the timing circuit, for example between groundand the lower terminal of the capacitor 329.

It will be apparent to those skilled in the art that various changes inthe system and the components thereof may be made. For example, thesystem might include an additional output circuit or circuits, toindicate extremely light rain, or rain in a range lying between drizzleand normal rain; In this respect, any number of output circuits could beadded, provided of course that additional amplitude discriminators areprovided in the impact sensor circuit. And, if desired, a continuoussignal, proportional to the impact energy, may be used. Similarly,additional outputs may be taken from the mass accumulation sensorcircuit, the rebound sensor circuitY and the photo-optical circuit,provided additional amplitude discriminators are used.

It is also possible to simplify the system, particularly where it is notnecessary to provide output indications of all types of precipitation,or where a high degree of accuracy is not required. For example, themass accumulation sensor arrangement need not be used where anindication of freezing drizzle or freezing rain is not required.

It will be understood that other modifications and variations may beeffected without departing from the spirit and scope of the novelconcepts of this invention.

We claim as our invention:

l. In a precipitation sensing system, impact sensing means including asurface exposed to precipitation and electro-mechanical transducer meansfor developing control signals in response to impacts of particles onsaid surface, rebound sensing means including a surface exposed toprecipitation and an electro-mechanical transducer device in spacedrelation to said surface for developing control signals in response torebounds of particles from said surface, photo-optical means including alight source for illuminating a certain region through whichprecipitation falls and photocell means for deriving control signals inresponse to light impulses arising from reflection of light fromparticles falling through said region, mass accumulation sensing meansincluding a vibration device having a surface exposed to precipitationand means for developing control signals in response to a lowerednatural resonant frequency of vibration of said device caused byaccumulation of freezing precipitation on said surface, a plurality ofoutput circuits corresponding to various forms of precipitation, andlogic circuitry responsive to said control signals for selectivelyenergizing said output circuits.

2. In a precipitation sensing system, mass accumulation sensing meansincluding a vibration device having a surface exposed to precipitationand means for developing control signals in response to a lowerednatural

1. IN A PRECIPITATION SENSING SYSTEM, IMPACT SENSING MEANS INCLUDING ASURFACE EXPOSED TO PRECIPITATION AND ELECTRO-MECHANICAL TRANSDUCER MEANSFOR DEVELOPING CONTROL SIGNALS IN RESPONSE TO IMPACTS OF PARTICLES ONSAID SURFACE, REBOUND SENSING MEANS INCLUDING A SURFACE EXPOSED TOPRECIPITATION AND AN ELECTRO-MECHANICAL TRANSDUCER DEVICE IN SPACEDRELATION TO SAID SURFACE FOR DEVELOPING CONTROL SIGNALS IN RESPONSE TOREBOUNDS OF PARTICLES FROM SAID SURFACE, PHOTO-OPTICAL MEANS INCLUDING ALIGHT SOURCE FOR ILLUMINATING A CERTAIN REGION THROUGH WHICHPRECIPITATION FALLS AND PHOTOCELL MEANS FOR DERIVING CONTROL SIGNALS INRESPONSE TO LIGHT IMPULSES ARISING FROM REFLECTION OF LIGHT FROMPARTICLES FALLING THROUGH SAID REGION, MASS ACCUMULATION SENSING MEANSINCLUDING A VIBRATION DEVICE HAVING A SURFACE EXPOSED TO PRECIPITATIONAND MEANS FOR DEVELOPING CONTROL SIGNALS IN RESPONSE TO A LOWEREDNATURAL RESONANT FREQUENCY OF VIBRATION OF SAID DEVICE CAUSED BYACCUMULATION OF FREEZING PRECIPITATION ON SAID SURFACE, A PLURALITY OFOUTPUT CIRCUITS CORRESPONDING TO VARIOUS FORMS OF PRECIPITATION, ANDLOGIC CIRCUITRY RESPONSIVE TO SAID CONTROL SIGNALS FOR SELECTIVELYENERGIZING SAID OUTPUT CIRCUITS.